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While the continuity thesis arose in connection to abiogenesis, there is no reason for it to remain constrained there; the same reasoning applies to noogenesis or any similar key transition. Again, this is of key importance when using the continuity thesis as heuristics. If some empirical data or well-motivated theoretical model appears which would indicate discontinuity, we would be justified in rejecting the continuity thesis, of course. However, as long as such empirical or theoretical insights are lacking, we should be free to use it and investigate its consequences, even if some of them are extremely speculative and far-fetched.⁷

While the continuity thesis suggests that the transition from non-life to life is easier than a priori thought, it does not really prescribe where and how the transition did actually occur. It is still possible that early life came to Earth from somewhere else, in particular from Mars, which had perhaps been more conductive to abiogenesis than our planet at that epoch (e.g., [3.18]). The scenario in which abiogenesis first occurred on Mars and some early life forms were subsequently transported to Earth (while, presumably, Martian life either became extinct or remained in very limited enclaves after the environmental conditions there deteriorated 3.5–4 Gyr ago; see [3.37]) is as perfectly in agreement with the continuity thesis as are any of the conventional abiogenesis scenarios on Earth. Mutatis mutandis, other forms of panspermia, are consistent with the continuity thesis and the epistemological “machinery” behind it.

Now, there is an important consideration to take into account: while some panspermia could clearly occur naturally, and in an optimistic case, be effective very slowly over interstellar distances (see also the chapter by Balbi in this volume [3.7]), the constraints are much weaker in the case of its technogenic version, directed panspermia. The latter has been suggested by two titans of biochemistry, Francis Crick and Leslie Orgel, as a halfserious solution for multiple problems facing origin of life research: maybe our planet has been seeded, intentionally or not, with early life forms by an advanced technological civilization [3.17]. Directed panspermia is often made to sound like science fiction—which should not be taken pejoratively in the first place—although it is a scientifically legitimate hypothesis or a class of hypotheses. Critics have charged that it is untestable, although it is at least doubtful whether it is indeed so, or we should emancipate from the common myopia inherent in human short-term timescales in epistemology as well.

Figure 3.1 A symbolic representation of the feedback created by directed panspermia. It appears in addition to all other feedbacks studied by astrobiologists such as Chopra and Lineweaver [3.12].

The central feedback loop of directed panspermia is schematically shown in Figure 3.1. It seems uncontroversial that simple life is a precondition for complex life in both causal and chronological senses, and ditto for complex life in relation to advanced technological civilizations, such as those sought for in SETI searches. (Arrows in the figure are meant to relate primarily to the causal ordering.) While those issues are uncontroversial, the same cannot be said for the question what exactly can advanced civilizations act on, through one form of directed panspermia or another. We may imagine that this applies to a prospective habitable planet, like the Hadean Earth, in accordance with the original scenario of Crick and Orgel, but there is no reason not to consider other kinds of potential habitats. It is crucial to emphasize the potentiality here, since we now know that habitability in the medium to long term depends on whether a site is indeed inhabited or not, due to multiple complex biotic feedbacks [3.12]. This means that there was nothing obvious in habitability of Earth either—when observed from the Hadean temporal vantage point. On the contrary, something like very precise and advanced astrobiology and ecology had been required at the time (on behalf of potential Seeders) to predict even vaguely what could the hypothetical terrestrial biosphere evolve into in billions of years hence.

Taken together with the ever-increasing and at present inconceivable capacities of advanced biotechnology, this means that we can hardly specify where and how successful could the intentional seeding with life be. Hence, it seems appropriate to label the substrate where both local abiogenesis (in the Oparin-Haldane manner or any of its subsequent elaboration) and directed panspermia occur as “generalized habitable substrate” as a category wider than the usual—and rather narrow—set of habitable terrestrial planets. It is not, in the spirit of the words of Biblical Job cited above, enough to see “the waste and desolate land” in order to “make the ground put forth grass”.

Clearly, concerns of Copernicanism should be taken into account here: should we consider ourselves typical as far as capacity and intentionality for seeding other worlds are concerned? Obviously, the problem here is that we are creatures of our epoch and cannot observe our species “in the fullness of time”. We can speculate, though, on the basis of some actual trends—and indeed we should, taking into account high relevance of the issue.

First, let us consider the unintentional form of directed panspermia. Since 13 September 1959, when Luna 2 impacted the Moon, humans have been bringing their artifacts in contact with other celestial bodies—more than 60 years by now. During that period, great advances in microbiology have demonstrated how resistant terrestrial bacteria are and how achieving complete sterilization of any human artifact is practically impossible. Although people have been aware of this issue for quite some time (and the Office of Planetary Protection has been pompously set up at NASA), it is reasonable to wonder whether humans have already seeded other cosmic bodies with terrestrial life forms—or will do it soon in the continuation of our space programs. In particular, this is relevant for Mars, which is now home to a wide variety of human landers and rovers, and it is a target for future crewed missions. While attempts of sterilizing probes may indeed decrease chance of such an unintentional directed panspermia, sterilizing human astronauts is clearly impossible (and would be a criminal offense even if it were possible!). It is not just Mars—we cannot be sure that even the effectively interstellar probes like the Pioneers and the Voyagers do not carry minuscule pieces of Earth’s biosphere toward the stars. The Office of Planetary Protection notwithstanding, it is highly doubtful that anything short of complete cessation of all astronautic efforts can, in fact prevent such an unintentional directed panspermia. (I call it unintentional since, obviously, propagation of the terrestrial life was never a conscious idea; clearly, the outdated prejudices about fragility of microorganisms encouraged a somewhat relaxed approach to the possibility, which is still discounted as “science fiction” in some circles.) Contrariwise, if our efforts in space gain extent and momentum, the probability for this kind of directed panspermia will certainly increase with time, even if protective measures are applied (cf. [3.54]). The increase in probability will occur on timescales of human culture which are extremely short in comparison to timescales for astrophysical, as well as biological evolution. We shall return to this important point later.

Arguably, this kind of directed panspermia—an unintentional consequence of space activities of technological civilization—was not what Crick and Orgel had in mind. Intentional seeding of other habitats is certainly more interesting, but also more speculative, for obvious reasons. There is a parallel here with the transmission of non-native plants and animals between continents on Earth by humans. Obviously, humans transmitted useful crops like potato or maize from one continent to the rest of the world; equally obviously, transmission of various pathogens or the ten-lined potato beetle was unintentional and brought immense harm to humanity. Let us suppose that an advanced technological civilization will be capable of clearly separate the intentional from unintentional and to regulate the unintentional with 100% efficiency. Do such societies engage in intentional seeding of other worlds?

Again, we should try to take a look into the future of humanity first. There have been speculations and suggestions from time to time that humans could and indeed should seed other planets with the terrestrial kind of life. The reasoning is often based on a form of biocentric ethics: since planet with life is inherently more valuable than a dead planet, we ought to intervene when we encounter a dead—but potentially habitable—planet.⁸ The strongest contemporary proponent of directed panspermia and our ethical obligations of seeding the universe with life has been the American physical chemist Michael N. Mautner. In a series of papers and books, he has promoted the view that we have a duty to spread life throughout a presumably (mostly) dead universe [3.46–3.48]. Even more, he founded The Panspermia Society in 1995, with the explicit goal of bringing this moral imperative to practical fruition. Since the astrobiological revolution started about the same time, all this has resulted in much more vigorous discussions of the relevant technical and bioethical topics.

We cannot enter here into the latter aspect of the problem: as much as Mautner and others have argued that it is in fact our moral duty to seed other worlds [3.48], there are many astrobiologists and bioethicists who argue to the contrary (e.g., [3.49, 3.52]). This bioethical dilemma will remain with us for quite some time, one may safely presume. There is, however, one angle which has not been sufficiently discussed so far, namely, that the outcome of the debate has a political, as well as ethical, aspects. Even today, it seems plausible that private actors—for instance, companies such as SpaceX—can, if they so desire, launch proverbial soda-cans full of bacteria directed at Mars, Europa, or even some of the recently discovered extrasolar planets. Barring global nuclear war, or some other existential cataclysm, it is virtually certain that the launch mechanisms will become cheaper and more widely accessible in the future. We are not talking about some astronomically distant future, but the future unfolding on the timescales of years, decades, or at most centuries from now. There has been some revival of interest in all forms of panspermia ideas recently (e.g., [3.7, 3.32, 3.57]), which would hopefully give us better insight into the constrains and requirement for the long-term viability of any simple life forms in a variety of cosmic environments. Also, it could be easily shown that directed panspermia can be easily incorporated into a general category of numerical simulations in astrobiology [3.21].

However, if we wish to avoid the pitfall of chronocentrism, we have to take into account the “unthinkable”, namely, to speculate about what the future of human civilization or the past/present of advanced extraterrestrial civilizations may contain. This is not a luxury and it is not optional. The temporal scales of astrophysics and evolution are so much greater than those of human culture, so there are many temporal viewpoints of which we have no historical experience, even if we neglect all other parameter differences. While we cannot be certain that the same is valid for observers in other cosmic civilizations, it is reasonable to assume that a similar relationship between astrophysical and cultural timescales exists in at least that segment of civilizations emerging in physical conditions similar to those on Earth.⁹

Capabilities of advanced technological civilizations are, of course, unknown at present. However, it is exactly the reason why we are justified in using a general heuristics like the extended continuity thesis in thinking about them. Together with other general principles guiding our thinking (e.g., scientific realism, naturalism, and Copernicanism), we are free to modify or abandon them as the empirical data comes in—whenever that occurs. To insist that because we do not possess such data at present, in an early epoch of both our own and cosmic evolution, we should censor our thinking, is to succumb to chronocentrism of the worst sort.¹⁰

The other key input is the rate of habitat formation in the course of the Galactic history. The pioneer study of Charles Lineweaver [3.42]—and the subsequent improvements by [3.9, 3.43, 3.64]—indicates that the median age of Earth-like planets in the Galaxy is

which is significantly greater than the Earth/Solar System age. Since roughly that epoch, the rate of formation of terrestrial planets in the Milky Way has steadily decreased. While this can be used to strengthen Fermi’s Paradox (as argued at length in [3.14]), there are other interesting speculative applications of the Lineweaver timescale.

Notably, this timescale justifies our previous conclusion that we are living in an early epoch of evolution of our own brand of intelligent observers. The fact that we have discovered these timescales now and have our, even very crude and simplistic, models of the relevant processes implies that older and more advanced civilizations have much better and precise insight into the same. Thus, a Kardashev’s Type 2.x civilization is likely to be able not only to survey all sites for abiogenesis in the Galaxy but is likely to be able to predict the emergence of future habitable sites. Moreover, an implication of the research of Lineweaver and others is that, as far as terrestrial planet formation is concerned, the Milky Way is past its prime: the rate of such habitat formation already decreased and will continue to decrease in the future. A clear implication is that the rate of local occurrences of abiogenesis is decreasing and will continue to decrease.

This happens due to the processes of chemical evolution and star formation which we have studied. However, it also happens due to another, intuitively obvious, but so far not quantitatively studied process: the emergence of life and intelligence itself at individual sites in the Galaxy implies that the available number of sites for future emergencies of this kind is decreasing. In other words, if life emerges somewhere and persists, there are less “slots” left for such future emergencies in the Galaxy as a whole. This effect may or may not be very small so far—its magnitude depends on the intrinsic probability of local abiogenesis (averaged appropriately). While the continuity thesis tells us that this intrinsic probability should not be 10⁻¹⁰⁰, we cannot say whether it is 10⁻⁶, 10⁻³, 0.1, or 0.9, which the magnitude of the “filling the slots” effect, as manifested so far, hinges upon. However, what we may be rather certain is that in the future the effect will gain in importance. Especially as the number of habitable “slots” intrinsically falls off.

Of course, we need to keep in mind the point emphasized by Freeman Dyson (e.g., [3.24, 3.25]) that not all potential habitats are Earth-like planets; however, formation rates of other potential habitats generally follow the same pattern, which follows from the general chemodynamical evolution of the Milky Way. This was valid for the Galactic past, on which our evidence is based. Therefore, we are justified in using Earth-like planets as synonymous with habitats for origination and evolution of life and intelligence so far. Why is the temporal qualification important here? This is the key point here: because directed panspermia, in contrast to other processes leading to the expansion of life, is not bound by a narrow class of Earth-like planetary habitats.

It is obvious why this is so. The design space of advanced technological evolution (or what has been dubbed postbiological evolution) is many orders of magnitude larger than the design space of natural, biological evolution; for instance, even a primitive technological civilization such as this on Earth has already created more diverse ways of locomotion or perception than the animal evolution has managed to do since the Cambrian Explosion. Again, barring an existential disaster, all researchers in the futures studies agree that the transformative technologies of the near future, such as nanotechnology and AI will immensely expand the reach of (post)human design [3.11]. There is no reason to doubt, in accordance with the principles of exploratory engineering ([3.5, 3.22, 3.63], esp. Figure 3.1 in [3.5]), that what applies to the future of humanity will apply to other Galactic civilizations as well, notably the older and more advanced societies [3.19, 3.20]. After all, we share the same laws of physics and therefore the realm of the possible, although huge, is limited by the same constraints of physics (and, presumably, economics in a generalized sense; cf. [3.60]). Therefore, advanced societies are likely to be capable of creating life forms capable of surviving in other habitats, similar to the “Dyson tree”: a hypothetical plant (perhaps resembling a tree) capable of growing on the surface or inside a cometary nucleus (e.g., [3.25, 3.62]). Gradually, such organisms could be able to create the critical amount of complex organics to establish the basis for subsequent human colonization of objects such as those in the Kuiper Belt, which contain huge amounts of volatiles such as water, methane, or ammonia. Even more resistant organisms could be engineered to survive and thrive in the atmospheres of gaseous giants, along the classical suggestions of Sagan and Salpeter [3.55]. Of course, that visionary study suggested searching for naturally evolved ecologies in atmospheres of Jupiter and other gas giants. If such ecologies are physically possible, there is no reason to doubt that, even if they did not evolve naturally due to contingency and opportunism of evolution, they could still be engineered by sufficiently advanced civilizations.

Such nonstandard habitats—like cometary nuclei and gas giants’ atmospheres—are immensely larger than surfaces of Earth-like planets by any meaningful metric (e.g., number density per unit Galactic volume or the amount of available resources). Therefore, if some life forms could enter those niches, they are likely to dominate the overall cosmic tally of life forms—in the fullness of time, even if not up to now.¹¹ In other words, a randomly chosen living being in our universe is mostly likely to have directed panspermia in its phylogenetic past. Taking into account both the decreasing rate of the terrestrial planet formation with cosmic time, the perspective of those nonstandard habitats for engineered, if not naturally emerging biospheres, and the ideas of visionaries and optimists like Constantine Tsiolkovsky or Gerard O’Neill about entirely artificial habitats [3.4, 3.58]—all those point in the direction of a radically different astrobiological landscape than is conventionally assumed. As in many other stages of history of science and philosophy, it is the lack of imagination which is by far the bigger problem than its excess.